The ECC83 high impedance amplifier triode was designed specifically for audio use and has a high amplification factor of 100. Mullard describe the ECC83 as a high μ double triode, having separate cathodes, primarily intended for use as a resistance-coupled amplifier or phase inverter.
This valve is still found in todays designs of hi-fi audio amplifiers. In the classic Mullard designs of the 1950s and 1960s the valve's two halves were used as second stage amplification and cathode coupled phase splitter.
The phase splitter being required so that the push-pull output stage was driven correctly.
The thin glass tube envelope is 21 mm in diameter and, excluding the B9A base pins, is 48 mm tall.
References: Data-sheet, 3002 & 1040. Type ECC83 was first introduced in 1954.